(1) Field of the Invention
This invention relates to electromechanical vibrators and, in particular, to audio-frequency electromechanical vibrators adapted for a body-felt vibration reproduction in sound reproducing systems.
(2) Description of the Prior Art
A sound reproducing system has been known in the prior art as disclosed in, for example, U.S. Pat. No. 4,064,376, which reproduces from an electric signal not only sound heard by ear but also mechanical vibration of, preferably undertones lower than 150 Hz, to be directly transmitted to a body. Such a system has an electromechanical vibrator for reproducing the mechanical vibration which is fitted to a bed or a chair. An audio signal to be fed to sound reproducing speakers is also applied to the vibrator, preferably after passing through a filter for removing a higher frequency component than 150 Hz. A person on the chair or bed feels vibration while enjoying music from the speaker through ear.
An electro-dynamic tranducer is used for the vibrators in such sound reproducing system, a known vibrator has an arrangement similar to an electro-dynamic speaker, as shown in U.S. Pat. Nos. 4,064,376 and 4,354,067. The known vibrator has a magnetic circuit constituted by one or two permanent magnets and a magnetic yoke with a magnetic gap in which a drive coil is loosely fitted or disposed. The electric signal is applied to the drive coil and therefore, the coil and the yoke are moved relatively to cause the vibration.
In the known vibrator, the drive coil is disposed in a small magnetic gap and therefore strong vibrations can not be generated. Furthermore, a thin and good heat-conductivity case cover is required to make good heat radiation from the drive coil. This means that the case cover tends to vibrate in response to a higher frequency component included in the audio signal applied to the vibrator, so that the vibrator makes noise. In order to prevent the noise generation, the filter must be used to remove the higher frequency component from the audio signal applied to the vibrator.